1. Field of the Invention
The present invention relates to an improvement in the structure of a monolithic multi-layer piezoelectric element, which comprises a sintered body obtained by laminating a plurality of ceramic green sheets provided with electrode patterns on the major surfaces thereof, and then cofiring the components of the laminated assembly thus obtained.
2. Description of the Prior Art
FIGS. 10 and 11 are perspective and sectional views showing a conventional plate type piezoelectric element 1 which is generally employed as a piezoelectric element utilizing counter extensional vibration. A polarized piezoelectric ceramic member 2 is provided on its upper and lower surfaces with electrodes 3 and 4, respectively. Thus, the desired counter extensional vibration can be obtained by applying a voltage to the electrodes 3 and 4. In such a plate type piezoelectric element 1, the resonance frequency can generally be controlled by lapping its end surfaces, or more generally its outer peripheral surfaces.
However, a piezoelectric element 1 of the type shown in FIGS. 10 and 11 is unsatisfactory when employed as, e.g., a second resonator for a ladder-type filter, which must have a low impedance.
A proposed solution to this problem is the monolithic piezoelectric element 11 shown in a schematic sectional view in FIG. 12. A plurality of stacked ceramic layers 11a to 11c are formed with inner electrodes 13b and 13c therebetween and then cofired. Outer electrodes 13a and 13d may be formed simultaneously with the inner electrodes 13b and 13c or applied separately after the initial firing. The electrodes 13a to 13d are electrically connected with each other and with end electrodes 15 and 16 in an alternating manner. The stacked ceramic layers 11a to 11c are polarized in the alternating directions indicated by the arrows in FIG. 12 by applying a voltage to the end electrodes 15 and 16. Then, by applying voltages to the end electrodes 15 and 16, the stacked layers 11a to 11c can be respectively expanded and contracted.
In the aforementioned monolithic piezoelectric element 11, the impedance can be made extremely low in comparison with the conventional plate type piezoelectric element, 1 while still maintaining a substantial size. However, the known method of lapping the end surfaces of the sintered body cannot be applied in this case to controlling a desired resonance frequency. In the monolithic piezoelectric element 11 as shown in FIG. 12, the end electrodes 15 and 16 formed on the end surfaces of the sintered body would be removed by lapping the end surfaces, preventing the use of the element 11 as a resonator.